1. Field of the Invention
This invention relates to an air/fuel ratio control system for an outboard motor internal combustion engine.
2. Description of the Related Art
Feedback control of the air/fuel ratio of an internal combustion engine using an O2 sensor (oxygen sensor or air/fuel ratio sensor) installed in the exhaust system is known to the prior art. This type of control utilizes a characteristic of the O2 sensor whereby the sensor produces an output whose property only changes near the stoichiometric air/fuel ratio. Feedback control is therefore not possible when the desired air/fuel ratio is not the stoichiometric air/fuel ratio. The practice has therefore been to calculate a learned correction coefficient from the feedback correction coefficient used by the feedback control and correct the basic fuel injection quantity using the learned correction coefficient when the desired air/fuel ratio is not the stoichiometric air/fuel ratio, thereby controlling, i.e., open-loop controlling the air/fuel ratio to one other than the stoichiometric air/fuel ratio, as taught, for example, in Japanese Laid-Open Patent Application No. Sho 57(1982)-105530.) It is also known to control the air/fuel ratio using a universal air/fuel ratio sensor that produces linear outputs proportional to the air/fuel ratio in a wider range.
If such a universal air/fuel ratio sensor is used, since it becomes possible to calculate the correction coefficient at a time when the desired air/fuel ratio is set to one other than the stoichiometric air/fuel ratio, the air/fuel ratio can be controlled more precisely than a case where the O2 sensor is used. However, since the universal sensor is more expensive than the O2 sensor, this is disadvantageous from the viewpoint of cost.